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Insulating a Log Cabin from the Exterior

Doublebit | Posted in General Questions on

We’re in the process of converting a traditional log cabin built in the 1950s with chinked 6″ round silver birch logs from a (zone 4, northern Maine) seasonal camp into a full time home. We would like to preserve the interior look of these aged birch logs by adding insulation to the exterior (walls and roof) and are looking for advice to do this intelligently. The interior logs would be left exposed.

My best thinking thus far (no claim to this as good thinking ;-)) is to first put vertical 2×4 sleepers (mounted edgewise) on the exterior walls. Second step would be to spray with closed cell foam to the full depth of the sleepers, then either add a vapor/rain barrier and then horizontal siding or skip the vapor barrier and go directly to the siding.

I’d also be interested in any advice you might have for the roof deck (esp. moisture control within the assembly) for which I’m planning a vapor barrier, two layers of polyiso sheets, Zip sheathing and then Grace I&W shield with a raised seam metal roof.

My main concern is avoiding moisture buildup and future mold/rot problems. I would love to hear any advice/counsel you could give me. Thanks, Dave

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Replies

  1. plumb_bob | | #1

    Insulating the outside of a log structure is a tough one. Downsides with your plan:
    1) Log structures have such a volume of wood in its natural state that movement induced by variations in humidity, temperature on other factors is a given. The CCSF will adhere to the logs and be air tight for a while, but as the structure moves this bond will break, and will no longer be air tight.
    2) You risk warm moist air leaking from the interior of the structure to the space between the exterior of the logs and the insulation, where it could condense into water, causing decay. This is theoretical, it is hard to anticipate what will happen in practice.

    I would consider framing the walls at the exterior as you plan, then using roxul or similar for insulation. This is vapour open so should not cause a moisture issue.

    If you plumb the corners, and then run strings between the plumb lines, you can get very nice straight exterior walls. I would ensure the cavity created is impervious to rodents and insects.

    1. Doublebit | | #11

      Plumb-bob - Thanks you. You have convinced me that spray foam is not the answer as you are correct - those walls DO move! You must have wrestled with log-cabins (re: your comment on nice straight walls)!

  2. Expert Member
    Akos | | #2

    I don't see why you would need spray foam at all.

    I think the simplest would be to simply frame a new wall just outside the logs. This can be set onto a steel angle lagged to the foundation. The wall would be detailed like any standard wall with taped sheathing as the air barrier. As Plumb-Bob pointed out, critter protection is key for these, so make sure there are no gaps anywhere, this might mean some extra framing, plywood and wire mesh around edges.

    The wall could be either 2x4 or 2x6 and insulated with regular batts. A 2x6 wall combined with 5" or so of logs is about an R23 assembly which is excellent in your climate. The 2x4 is less but also good enough.

    The other option is to cover the logs with rigid insulation with the seams taped held in place with 1x4 strapping screwed to the logs. The siding can than be attached to the strapping. Again, key here is detailing all openings to prevent critters from getting in. This means perforated flashing on the bottom and picture framing the sides and top with 2x on edge ripped to height. About R15 of rigid is a good target, a bit less and I would not loose sleep over it.

    In either case, you have to be careful with your windows as the sill has to be extended past the new siding. Might make sense to install new windows that are mounted onto the outside face of the new wall.

    1. Doublebit | | #10

      Akos - Thank you for that detailed response. As you suggest, we are contemplating new windows for this renovation.

      1. Expert Member
        Akos | | #16

        If your logs are reasonably flat, one quick retrofit might be Zip insulated panels (R9 or R12) attached directly to the logs. Not cheap but goes on quick, you get insulation, vapor control, sheathing and WRB in one product. Once this is on, the rest of the build (windows, doors, siding) is pretty much standard build with a bit of the extra interior trim work.

    2. maine_tyler | | #19

      "R23 assembly which is excellent in your climate."
      Are you stating that for zone 4 or for Northern Maine? Northern Maine is actually zone 7 (or possibly 6)

  3. DC_Contrarian | | #3

    Think house-within-a-house.

  4. Doublebit | | #4

    Thank you for those replies... you have my wheels turning. Yes, the log movement is a fact of life in that log wall. The chinking is loose and can't really be tightened up permanently because of that movement.

    So if I purse the batt idea then in the winter there will be water vapor moving from the interior and condensing somewhere inside the batt insulation? Even if I have a good vapor permeable barrier on the outside (and it will obviously breathe to the inside), won't the batts get moldy and soaking wet leading to rot of the interior and exterior wall assemblies?

    Is this "house-within-a-house" identical to what would be done with a double-wall framed assembly?

    1. DC_Contrarian | | #5

      Answer to the first question: vapor moves from warm to cool and from moist to dry. In most places, exterior walls have one side that is normally warm and moist and the other side is cool and dry. In Maine, the warm side is the interior (in Miami it's the exterior). The simplest way to build a wall is with a vapor barrier on the warm, moist side, then insulation, and the cold, dry side vapor-open to dry to the exterior. The vapor barrier keeps moisture out of the wall, and any that gets in gets driven to the exterior.

      Second part: "House-within-a-house" is my stock answer when someone says they have a cool old building that they want to keep the character of but bring up to modern standards.

      Putting those two parts together you want to have the legacy wall, then a vapor barrier, then insulation, then an outside wall that is able to dry to the exterior.

  5. plumb_bob | | #6

    The log wall will act as the vapour barrier, so water vapour should not transfer through the wall by diffusion. However, log walls are poor air barriers, so water vapour will move through the wall through air leakage, which is why you want something vapour permeable on the outside.

    1. Doublebit | | #9

      I'm still unclear as to whether that water vapor drive from the interior will condense within the wall before it can diffuse through the vapor permeable outside membrane in very cold temperatures.

  6. Doublebit | | #7

    Ah, DC, thanks for the clarification - got it. What would you think about using foil faced polyiso (facing inward) as the vapor barrier and insulating layer. We can readily get 2nds of polyiso. In any case would I have to worry about that vapor drive to the exterior condensing on the foil face or within a batt? It gets seriously cold here...

    1. DC_Contrarian | | #8

      If you taped the seams that would work.

  7. Expert Member
    MALCOLM TAYLOR | | #12

    Doublebit,

    The devil will be in the details around openings. The new walls need to be attached to the existing structure so that the logs can still move seasonally. That's not too difficult, using slides as you do with the windows and doors on log buildings.

    Where things get tricky is at openings where the two walls intersect and need to be kept waterproof, and at the eaves which need some expansion room built in so the roof doesn't end up being supported by the new non-load bearing walls.

    1. Doublebit | | #13

      Well, I'm the detail guy as I'm aging out of "prime mover" status, so I'll have no one to blame but myself if I don't get those right. This is my first encounter with a log building, so I truly appreciate the knowledge that you are sharing with me. I don't even know what a slide is (thank you for that bit), but I'm going to find out in short order. Do you have any suggestions for how to do this for the eaves and intersections?

      This is a unique building in that it was made with veneer-grade silver birch hearts. They came from a now-defunct local veneer mill. They were what was left over when the log was turned to its minimum diameter.

      1. DC_Contrarian | | #14

        So this where I go back to "house-within-a-house." Build a nice 2x6 exterior wall. Sheath it with whatever you use up there, and mount the doors and windows in that wall and flash them on the exterior. Then trim out the interior of the windows and doors so that they meet the logs and will be OK if the logs move around.

        Are you keeping the roof? There might be some tricky details where the walls meet the roof. Also, what kind of foundation is there, and how do the floors attach to the walls?

      2. Expert Member
        MALCOLM TAYLOR | | #15

        Doublebits,

        To compensate for the seasonal movement, log buildings have their door and window attached to the structure with slides - often just slots in the frames for the fasteners. They also need to have a gap left above them so they don't end up taking the weight of the structure as it shrinks. I once worked on a Panabode https://panabode.com where the front door ended up supporting the whole exterior wall of the cabin.

        As the outer wall will stay in one place as the logs behind move, it should be built with a gap between it and the soffits above so that it doesn't end up taking up roof loads when the logs shrink, and you need to attach the trim at the top of your siding to the roof above so it moves with the logs and slides up and down while the outer wall remains stationary.

        Dealing with the window and door openings is probably best done by following DC's advice and moving them to the outer wall where they won't move around.

        1. Doublebit | | #18

          Malcom, Thanks for the explanation of "sliding" - makes sense to me. Going to have to do a little head scratching on how to create that sliding trim at the top of the wall. We will be following DC's (and your) advice re: locating windows and doors in the exterior wall. My wife will like using the deep returns as shelves!

      3. Doublebit | | #17

        DC Contrarian - We plan to keep the roof but will remove existing soffit and fascia in the renovation. The "foundation is currently piers simply built on concrete disks (float on ground). Log walls sit on top of the floor joists.

        We are going to try to replace those floating piers with frost-posts if local regulations allow (we are a grandfathered property with respect to distance to the lake and may require a special exemption to do so)

  8. plumb_bob | | #20

    Settlement of log structures is caused by the logs shrinking as they dry, and also from the compressive forces of the dead weight of the logs squashing together. With green logs this can add up to inches on a 9' wall. Your log shell will have reached full settlement years ago, and you will only need to account for the seasonal movement of the logs.

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